The invention pertains to methods for manufacturing dielectric materials.
Barium strontium titanate (BST) is used in electronic devices, particularly in electronic devices such as capacitors, because of its high dielectric constant (between approximately 200 and 6,000). Other applications of BST include capacitor-varistors, positive temperature coefficient (PTC) resistors, thick film multilayer ceramic capacitors (MLC), embedded capacitance laminates, and bulk ceramic transducers. Thick films of BST have also been used in gigabit dynamic random access memory devices (DRAMs), in bypass capacitors, in filters, and in GaAs microwave monolithic integrated circuit (MMICs).
Practical applications of BST require inexpensive manufacturing processes that are applicable to small geometry devices. One method of maintaining a selected value of capacitance while shrinking the size of a capacitor is the use of small BST particles. Unfortunately, such small BST particles cannot be manufactured by conventional ceramic processes that produce BST particles that tend to stick together or agglomerate, increasing the effective particle size.
Conventional sputtering processes used to form BST films produce reproducible films, but the quality of these films depends on the quality of the sputtering target. In addition, the deposition rate of such BST films can be low, restricting the use of BST films to devices that are produced in low volumes. Improved BST sputtering targets are needed to achieve the higher deposition rates necessary for high volume production.
A significant factor in producing a satisfactory sputtering target is the quality of the BST powder used to make the target. The performance of a sputtering target depends on BST particle size, size distribution, chemical homogeneity, and sintering temperature. The sintering temperature of a BST target can be lowered by using small particles. Nanometer-sized BST particles are infrequently used because such particles are much more expensive than larger, agglomerated particles that have a wide range of particle sizes. Existing processes for nanometer-sized BST particles involve high temperature processing, expensive organo-metallic precursors, hazardous and lengthy process steps, and hazardous reagents, and produce irregularly shaped or agglomerated particles.
One prior art method of manufacturing BST particles is referred to as an oxalic precipitation method. In this method, Ba0.6Sr0.4TiO (OH)2C2O4xc2x74H2O precipitate is prepared by co-precipitation of cations as a mixed oxalate in an oxalic acid solution. The precipitate is calcined at 900xc2x0 C.-1000xc2x0 C. to convert the precipitate into BST. See, for example, P. K. Gallagher et al., xe2x80x9cPreparation of Semiconducting Titanates by Chemical Method,xe2x80x9d J. Am. Ceram. Soc., vol. 46, pp 359-365 (1963) and F. Schrey, xe2x80x9cEffect of pH on the Chemical Preparation of Barium-Strontium Titanate,xe2x80x9d J. Am. Cer. Soc., vol. 48, pp 401-405 (1965). The oxalic precipitation methods described in these references are complex, and the organic compounds used are expensive. In addition, the BST particles produced by these methods are irregularly shaped, strongly agglomerated, and are large, having dimensions of between 500 nm and 800 nm.
U.S. Pat. No. 4,677,083 discloses another method of preparing BST powders. In this method, a TiCl4 solution is mixed with barium nitrate and strontium nitrate at a pH of 14. This mixture is held at a temperature of 100xc2x0 C. for 4 hours, followed by drying at 100xc2x0 C. for 24 hours, forming a product that includes KCl and KNO3 as by-products. These by-products are removed and the remaining product is heat-treated at 800xc2x0 C. for 2 hours. This process requires removal of the KCl and KNO3 by-products prior to heat treatment. In addition, this process involves dilution of a TiCl4 solution at a high pH, increasing the cost of this method. The BST particles produced by this process are irregularly shaped and the particle size varies over a wide range.
Another process for the precipitation of BST particles includes synthesis of BaTi(C6H4O2)3 and SrTi (C6H4O2)3 by reacting titanium tetrachloride and C6H4(OH)2 in a toluene solvent with boiling water suspensions of BaCO3 and SrCO3, respectively. This produces aqueous solutions of BaTi (C6H4O2)3 and SrTi (C6H4O2)3 that are freeze dried, forming a mixture of solid complexes. The mixture is subjected to pyrolysis and calcination at 700xc2x0 C. for 2 hours. This process produces strongly agglomerated, irregularly shaped BST particles having dimensions of about 800 nm. This method and the BST particles produced by this method are discussed in N. J. Ali and S. J. Milne, xe2x80x9cSynthesis and Processing Characteristics of Ba0.65Sr0.35TiO3 Powders from Catecholate Precursors,xe2x80x9d J. Am. Ceram. Soc., vol. 76, pp. 2321-2326 (1993).
Another method of producing BST particles involves the synthesis of an oxalate precipitate, Ba1xe2x88x92xSrxTiO3(C2O4)2xc2x74H2O (wherein 0.0xe2x89xa6xxe2x89xa60.3), by mixing TiO(NO3)2, Ba(NO)2, Sr(NO)2, and a titanyl oxalate solution to produce a precipitate that is calcinated for 2 hours at 900xc2x0 C. This process is described in T. Noh et al., xe2x80x9cChemical Preparation of Barium-Strontium Titanate,xe2x80x9d Bull. Korean Chem. Soc., vol. 16, pp. 1180-1184 (1995). This method produces strongly agglomerated BST particles having dimensions of between about 13 nm and 40 nm.
In another method, BST particles are prepared by a vapor-phase hydrolysis of precursors obtained from alkoxide-hydroxide. This method involves preparation of Ba(OH)2xc2x78H2O and Sr(OH)2xc2x78H2O at 300xc2x0 C. to obtain precursors Ba(OH)2xc2x7H2O and Sr(OH)2, respectively. These precursors are dissolved in methanol mixed with Ti-isopropoxide in a dry N2 environment, and maintained for 15 hours at room temperature. The methanol is then evaporated, producing a powder precursor that is then slowly hydrolyzed at 100xc2x0 C. in an N2 environment. The hydrolyzed powder is then calcined at 900xc2x0 C. for 2 hours, and ball milled in alcohol for 24 hours to obtain BST particles. The BST particles produced by this method are strongly agglomerated and are not separable by ultrasonication. This method is described more fully in T. Hayashi et al., xe2x80x9cPreparation of Ba1xe2x88x92xSrxTiO3 Particles by Vapor-Phase Hydrolysis of Precursors Formed from Alkoxide-Hydroxide,xe2x80x9d Jpn. J. Appl. Phys., vol. 37, pp. 5232-5236 (1998).
In summary, solution-based BST powder synthesis methods involve difficult chemical precipitation steps, and produce BST particles that are large, irregularly shaped, or strongly agglomerated. In addition, these methods produce BST particles having varying chemical compositions and use expensive organic precursors and hazardous process steps. None of these methods produce acceptably sized and shaped BST particles and new methods and apparatus for the production of such particles are needed.
Methods form manufacturing nanometer-sized barium strontium titanate Ba1xe2x88x92xSrxTiO3 (BST) particles and particles of other materials, such as barium ferrites, metal oxides (e.g., iron oxide, tin oxide), and other semiconductors, insulators, ferrolectrics are provided. The methods include mixing BST particle components and matrix components that are melted to form a glassy matrix containing BST particles or precursors thereof. The BST particle components include oxides, hydroxides, and carbonates of barium, strontium, and titanium. Matrix components include materials comprising sodium, particularly sodium salts such as sodium borate (Na2Oxc2x72B2O3), sodium carbonate (Na2CO3), and sodium silicate (Na2Oxc2x7SiO2xc2x79H2O), selected according to a predetermined molar ratio. The mixture of particle components and matrix components is melted and the molten mixture is quenched in ice water or by another method to produce an amorphous material, typically in the form of flakes or other irregular solids. The amorphous material is annealed to form BST particles in the amorphous material. The BST particles are separated by exposing the flakes or other solids to an acidic solution (or other solution) that dissolves or otherwise removes a glassy matrix portion of the flakes.
In representative embodiments, methods of manufacturing a powder of a dielectric compound include selecting particle components combinable to form the dielectric compound and matrix components. The particle and matrix components are combined to form a mixture that is melted to produce a glassy matrix containing the dielectric compound or a precursor thereof. The molten mixture is quenched to produce solid material and the solid material is annealed to produce BST particles in a glassy matrix. The BST particles are removed from the glassy material by exposing the glassy matrix to a solution that removes the glassy matrix. In representative embodiments, the dielectric compound is Ba1xe2x88x92xSrxTiO3, wherein 0xe2x89xa6xxe2x89xa61. In further embodiments, the particle components are selected from a group consisting essentially of oxides, hydroxides, and carbonates of Ba, Sr, and Ti and the matrix components are selected from the group consisting of Na2Oxc2x7SiO2xc2x79H2O, Na2Oxc2x72B2O3, and Na2CO3.
In additional embodiments, the particle components are selected from the group consisting essentially of barium titanate (BaTiO3), strontium titanate (SrTiO3), barium carbonate (BaCO3), strontium carbonate (SrCO3), and titanium oxide (TiO2) and the solution that separates the dielectric compound from the glassy matrix is an acidic solution.
In additional embodiments, a molar ratio of particle components to matrix components is selected so that the solid material produced by quenching includes an amorphous glassy matrix. In specific embodiments, a molar ratio of the particle components is selected based on a Ba fraction of the BST. In a representative embodiment, the molar ratio is between about 0.40 and 1.60.
In particular examples, the matrix components consist essentially of 1.34 part by mole Na2Oxc2x72B2O3, 1.13 parts by mole NaCO3 Na2CO3, and 1.0 parts by mole Na2Oxc2x7SiO2xc2x79H2O.
In other examples, the solid material produced by quenching is annealed by exposing the solid material to a temperature in a range of about 450xc2x0 C. to 700xc2x0 C. for a period of about 2 hours.
In yet other examples, the matrix components include one or more of zinc oxide, zinc carbonate, or zinc hydroxide.
In some examples, the mixture of the particle components and matrix components is melted by exposing the mixture to a temperature in the range of about 1100xc2x0 C. to 1,700xc2x0 C.
Capacitors comprising a dielectric layer comprising BST particles having dimensions of less than about 80 nm, or preferably, less than about 40 nm are provided. Sputtering targets are provided that comprise BST particles having dimensions of less than about 80 nm, or preferably less than about 40 nm.
Methods of producing a dielectric powder comprise mixing powder components and matrix components to form a mixture. The mixture is melted and then cooled to form a glassy material. The glassy material is annealed to form the dielectric powder that is separated from the glassy material by exposing the glassy material to an acidic solution. In some embodiments, the annealing step includes annealing at a first temperature and a second temperature, wherein the first temperature is in a range of about 400xc2x0 C. to 550xc2x0 C. and the second temperature is in a range of about 550xc2x0 C. to 700xc2x0 C. In additional embodiments, the powder components consist essentially of compounds of Ba, Sr, and Ti and the matrix components consist essentially of Na2Oxc2x72B2O3, Na2CO3, and Na2Oxc2x7SiO2xc2x79H2O. In other representative embodiments, a molar ratio of the powder components to the matrix components is selected so that the glassy material includes an amorphous material formed by the matrix components.
A solid material for producing BST is provided that includes a glassy matrix and BST precursors. In representative embodiments, the glassy matrix is produced by melting one or more of the group consisting essentially of Na2Oxc2x72B2O3, Na2CO3, and Na2Oxc2x7SiO2xc2x79H2O and the BST precursors consist essentially of oxides, hydroxides, and carbonates of barium, strontium, and titanium.